This invention relates generally to an exposure apparatus for projecting pattern of a mask onto a photosensitive substrate through a projection optical system. More particularly, the invention concerns an exposure apparatus of the type that uses ultraviolet light as exposure light.
Conventionally, the procedure of manufacturing semiconductor devices comprising very fine patterns such as LSI or VLSI uses a reduction type projection exposure apparatus which functions to project and photoprint, in a reduced scale, a circuit pattern formed on a mask onto a substrate being coated with a photosensitive material. Increases in the density of the semiconductor device have forced further miniaturization, and advancement in the resist process as well as improvement of the exposure apparatus to meet the miniaturization have been pursued.
The resolving power of an exposure apparatus can be improved by, for example, shortening the exposure wavelength used or by enlarging the numerical aperture (NA) of a projection optical system.
As regards the exposure wavelength, recently, in place of i-line (365 nm), KrF excimer lasers having an emission wavelength of about 248 nm or ArF excimer lasers having an emission wavelength of about 193 nm have been developed. Further, fluorine (F2) lasers having an emission wavelength of about 157 nm are being developed.
In regards to deep ultraviolet light, particularly, ArF excimer lasers having a wavelength of about 193 nm or F2 excimer lasers having an emission wavelength of about 157 nm, it is known that there is an oxygen (O2) absorption band in the regions about these wavelengths.
For example, because of its short wavelength (157 nm), the applicability of fluorine excimer lasers to exposure apparatuses have been attempted. However, the wavelength of 157 nm is present in a range of a wavelength region generally called a vacuum ultraviolet region. In such a wavelength region, the absorption of light by oxygen molecules is large. As a result, the atmosphere does not transmit most of the light. Therefore, this light source can be used only in a situation that the atmospheric pressure is reduced close to vacuum and that the oxygen concentration is sufficiently lowered. According to xe2x80x9cPhotochemistry of Small Moleculesxe2x80x9d, Hideo Okabe, A Wiley-Interscience Publication, 197, pp. 224-226, the absorption coefficient of oxygen to light of a wavelength of 157 nm is about 190 atmxe2x88x921cmxe2x88x921. This means that, when light having a wavelength of 157 nm passes through a gas having an oxygen concentration of 1%, under a unit atmospheric pressure, the transmission factor T per 1 cm is only:
T=exp(xe2x88x92190xc3x971cmxc3x970.01atm)=0.150.
Further, as the oxygen absorbs the light, ozone (O3) is produced. The ozone accelerates the absorption of light, and the transmission factor is degraded considerably. Additionally, various products are created by the ozone, and they are deposited on the surfaces of optical elements to decrease the efficiency of the optical system.
In consideration of them, in projection exposure apparatuses which use deep ultraviolet rays such as an ArF excimer laser or an F2 excimer laser, purging means such as inactive gas (e.g., nitrogen) is provided at the light path of an exposure optical system, to suppress the oxygen concentration at the light path to a low level on an order of a few ppm or less.
As described above, in exposure apparatuses using deep ultraviolet light, particularly, an ArF excimer laser having a wavelength of about 193 nm or a fluorine (F2) excimer laser having a wavelength of about 157 nm, because the ArF excimer laser light or F2 excimer laser light is very easily absorbed by a substance, the light path must be purged to a level on an order of a few ppm or less. Further, this is also the case with the moisture. It should be reduced to a level of a ppm order.
In order to assure the transmission factor to ultraviolet light or the stability of it, the light path of the ultraviolet light in relation to a reticle stage, for example, of an exposure apparatus may be purged. As an example, Japanese Laid-Open Patent Application, Laid-Open No. 260385/1994 shows blowing an inactive gas against a photosensitive substrate. However, this is insufficient in regard to the purge of oxygen and moisture. Japanese Laid-Open Patent Application, Laid-Open No. 279458/1996 shows use of a sealingly covering member for covering the whole space, adjacent to a photosensitive substrate, below a bottom end of a projection optical system. This structure is, however, impractical because the stage motion is made difficult.
As described above, in exposure apparatuses using ultraviolet light, particularly, ArF excimer laser light or F2 excimer laser light, because of a large absorption of the ArF excimer laser light or F2 excimer laser light by oxygen or the water content, it is necessary to lower the oxygen concentration and water concentration so as to obtain a sufficient transmission factor and a stability of the ultraviolet light.
It is, therefore, desired to develop purging means effective to an ultraviolet light path in an exposure apparatus, particularly, in portions adjacent to a wafer and a reticle where insertion and retraction of a component are made frequently.
It is an object of the present invention to provide purging means suitably usable in an exposure apparatus for projecting a pattern of a mask onto a photosensitive substrate, which is effective to locally purge an exposure light path of the exposure apparatus by use of an inactive gas.